I've discovered why I prefer the Gen4 grip to the Gen3 grip--the 4's is smaller, and now I know how much smaller.

I measured the depth--front to back--of my 31G4's grip compared with my 32G3's. I measured at right angle to the 'vertical' line of the front of the grip, in the bottoms of the finger grooves.

The G4's grip at the top groove is about 0.040" shorter, at the 2nd groove it's about 0.090" shorter, and at the 3rd (bottom) groove it's about 0.10" smaller. A tenth of an inch (and less) is not much difference, but it's enough for me to prefer the feel of the G4 grip.

I did not measure the add-on backstraps; I didn't even see if they're in the box.